Molybdenum complexes azulene

Molybdenum hexacarbonyl reacts with azulenes to give complexes of the type [Mo2(CO)6(azulene)], of unknown structure (29). 

It might be expected that azulenes would be able to act as 7-electron ligands. In those complexes whose crystal structures have been determined this is not so. In the binuclear iron complex, (azuIenejFe CCOjj, for example, azulene acts as a 5 + 3 electron ligand while in the similar molybdenum compound, (azulenejMo CCO), it is a 2 x 5 electron ligand. 

---